Wear bushings are used while drilling to protect the bore of a surface or subsea wellhead from damage or wear. The bore of the wellhead must be protected so that metal-to-metal seal assemblies can later be installed when a casing hanger is landed and the annulus between the wellhead bore and the casing run is “packed-off” with the metal seal assembly. Scratches in the wellhead bore would prevent the metal seal from passing pressure tests.
After a casing string is run in the wellhead, a special trip is typically made to install a shorter wear bushing with a smaller bore (via drill pipe) above the casing hanger to again protect the wellhead. Drilling can then commence again.
When subsea wellheads were initially introduced, they were used in shallow water depths, e.g., 300 feet (ft.)-500 feet (ft.). The time required to install (trip in) and retrieve (trip out) a wear bushing for such shallow depths was short. One trip was performed via drill pipe just to install the wear bushing and a separate trip was performed via drill pipe just to retrieve the wear bushing.
Later, designs of wear bushings were introduced to install and retrieve wear bushings while the drill bit and bottom hole assembly were being run and retrieved. These were called “bit runable wear bushings” and later shortened to “wear sleeves.” A drawback of these designs is that every time the bit is retrieved, the wear sleeve is also retrieved. There is still risk involved with this process, however, because the drill bit passes through the exposed wellhead before the wear sleeve is installed and, on the trip out, the wear sleeve is retrieved first, and later the drill bit will pass through the exposed wellhead. In the case of the smaller, deeper drilling intervals, the drilled hole may require multiple trips because worn drill bits would have to be replaced before the desired depth of penetration is reached.
United States Patent Application Publication No. U.S. 2002/0092656 proposes another solution, which employs a selectively retrievable wear sleeve. The invention is a running tool having an enlarged diameter section fitted with a gripping mechanism designed to engage selectively the wear sleeve. The gripping mechanism is activated with hydrostatic pressure created using a plug, which engages a shoulder formed on the inner diameter of the running tool just below the gripping mechanism. A drawback of this design is that the plug blocks the ability to circulate the drilling fluid (mud) down hole. Thus, when the plug is dropped down the drill string, the plug then lands on a shoulder and the retrieval process begins. If, at this time, a well kick occurs and the blowout preventer (BOP) rams are closed, the operator is not permitted to circulate out the kick by pumping down the drill pipe and taking returns up through the Choke and Kill lines located below the blowout preventer (BOP) rams. Such a case of not being able to circulate out the kick can lead to a dangerous situation. A further drawback of this design is that the enlarged diameter section of the tool, which is designed to travel down hole with the rest of the drill string during the drilling operation, is large enough to interfere with the inner diameter of the casing string and thereby potentially damage the casing string.